Apple Watch series 3

You can't add cellular to an apple watch,you have to buy the apple watch model with cellular built in,it's not something you can add on.

There are 2 versions of watch;

1.GPS only version and

2.GPS +CELLULAR version.

The Watch comes in two versions; non-cellular and cellular. If you didn't buy a cellular watch, it won't have it!

However, you can make phone calls from the watch using the Phone app on your watch. These calls are routed through your iPhone that the watch is paired to.

ASC - Late 2012 Mac mini, macOS 10.15.4; Watch, series 5 GPS/cellular, watchOS 6.2; iPad Air 2, ipadOS 13.4; iPhone 8+, iOS 13.4; TV 4th Gen, tvOS 13.4;  HomePods, audioOS 13.4 and Airport Express w/ Airplay 2

A Cellular Series 3 Apple Watch has a Red Dot 🔴 on the Digital Crown.

No Red Dot means it is a GPS only model.